Semiconductor transistors, in particular field-effect controlled switching devices such as a MISFET (Metal Insulator Semiconductor Field Effect Transistor), in the following also referred to as MOSFET (Metal Oxide Semiconductor Field Effect Transistor) and a HEMT (high-electron-mobility Field Effect Transistor) also known as heterostructure FET (HFET) and modulation-doped FET (MODFET) are used in a variety of applications. An HEMT is a transistor with a junction between two materials having different band gaps, such as GaN and AlGaN. In a GaN/AlGaN based HEMT, a two-dimensional electron gas (2DEG) arises near the interface between the AlGaN barrier layer and the GaN buffer layer. In an HEMT, the 2DEG forms the channel of the device. Similar principles may be utilized to select buffer and barrier layers that form a two-dimensional hole gas (2DHG) as the channel of the device. A 2DEG or a 2DHG is generally referred to as a two-dimensional carrier gas. Without further measures, the heterojunction configuration leads to a self-conducting, i.e., normally-on, transistor. Measures must be taken to prevent the channel region of an HEMT from being in a conductive state in the absence of a positive gate voltage.
Due to the high electron mobility of the two-dimensional carrier gas in the heterojunction configuration, HEMTs offer high conduction and low losses in comparison to many conventional semiconductor transistor designs. These advantageous conduction characteristics make HEMTs desirable in applications including, but not limited to, use as switches in power supplies and power converters, electric cars, air-conditioners, and in consumer electronics, for example. However, normally-on HEMTs have limited applicability in these applications because these devices must be accompanied by circuitry that can generate the negative voltages necessary to turn the device off. Such circuitry adds cost and complexity to the design. For this reason, it is typically desirable to include features in an HEMT that modify the intrinsic normally-on configuration and provide a device with a normally-off configuration (i.e., a positive threshold voltage).
One technique for providing a normally-off HEMT involves configuring the gate structure to modify the intrinsic conductive state of the 2DEG. For example, a relatively thick (typically 100 nm or greater) p-type doped GaN material can be formed under the gate electrode. This thick p-type GaN layer depletes the inversion layer under the gate structure, shifting the threshold voltage of the device to positive values. The p-type GaN layer must be thick enough to create a vertical field which depletes and populates the naturally occurring inversion channel underlying the barrier layer, typically a layer of AlGaN. The vertical field generated by a voltage applied to the gate electrode allows for on and off modulation of the inversion layer. However, the introduction of dopants into the gate structure of the device may determinately impact one or more device parameters, such as leakage current, maximum gate voltage and transconductance. Accordingly, there is a need to provide a normally-off HEMT without determinately impacting device parameters.